Forty-five workers will lose their jobs as part of a retrenchment at Boston Children's Hospital, which has been working to cut costs while maintaining the quality of its pediatric care amid accelerating changes in the Massachusetts health care marketplace. In all, the Harvard-affiliated hospital is eliminating 255 positions, about 2.6 percent of its 9,600 jobs, according to a memo sent to employees Wednesday by Children's chief executive James Mandell and president Sandra Fenwick. Some of those positions are now vacant, while other employees are being reassigned to take on new functions, the memo said, limiting the number of layoffs at the Boston medical center. The job cuts include both management and non-management positions, the executives said.